Building babl on Windows - Extensions problem

Hi,

as some of you might have learned from the last release announcement, it
should now be possible to compile babl on Microsoft Windows ("on" being
a MinGw+Msys-based build environment).

There's one problem remaining - right now, babl builds without
extensions and extension loading is disabled. This is because the
dlopen, dlsym and dlclose functions are used, which are not available on
the platform.

There are some options how this might be overcome:

1. Add win32 specific code

Either directly in the source files or as dl* replacements by some
autoconf magic. Probably doable, but feels like duplicated effort.